This paper presents new experimental developments of prompt self-powered neutron detectors (platinum, hafnium, gadolinium). In particular, we covered the Gd and Hf emitters with a small stainless steel tube called “the lining of the emitter,” to reach three goals: to decrease the deferred noise due to the sheath, reduce the current due to low energy external gamma-rays and stop a large part of the delayed β- rays, which are produced in the emitter. A large set of SPNDs with different thickness of lining was tested in the SILOE research reactor by means of two special dosimetry devices to study their neutron and gamma sensitivities, and their dynamic characteristics. Finally, we optimized three physically prompt neutron detectors (response times better than 0.03 s) with known sensitivities. We are now able to measure instantaneously in-line the thermal and epithermal neutron fluxes and the gamma field.